4,4&#39;-Diphenyl ether-dialdehyde-bis-dimethylacetal

ABSTRACT

Disclosed is the novel compound 4,4&#39;-diphenylether-dialdehyde-bis-dimethylacetyl of the formula ##STR1## and a process for its preparation from di-p-tolyl ether ##STR2## by anodic oxidation of the latter in the presence of methanol and of a supporting electrolyte. The anode material used in this reaction is preferably platinum, lead dioxide, graphite or vitreous carbon, and the cathode material used is preferably steel, nickel, or graphite. The supporting electrolyte used is preferably NaOCH 3 , KOH, KPF 6 , CsF, NaBF 4 , LiBF 4 , tetraethylammonium p-toluenesulfonate, H 2  SO 4  or CH 3  OSO 3  H, individually or as a mixture.

BACKGROUND OF THE INVENTION

The present invention relates to 4,4'-diphenylether-dialdehyde-bis-dimethylacetal and to a process for itspreparation.

It is known that anodic alkoxylation of unsubstituted or substitutedmethylbenzenes of the general formula (I) can lead to the correspondingbenzaldehyde-dialkylacetals of the general formula (II):

    ______________________________________                                         ##STR3##                                                                      ##STR4##                                                                     REACTANT  R.sup.1         R.sup.2 PRODUCT                                     ______________________________________                                        I a       H               CH.sub.3                                                                              II a                                        b         CH.sub.3        CH.sub.3                                                                              b                                           c         -- --OCH.sub.3  CH.sub.3                                                                              c                                                                     C.sub.2 H.sub.5                                                                        c'                                                    ##STR5##       CH.sub.3                                                                              d                                           e         -- --OCH.sub.2CHCH.sub.2                                                                      CH.sub.3                                                                              e                                           f                                                                                        ##STR6##       CH.sub.3                                                                              f                                           g         -- --Ot-C.sub.4 H.sub.9                                                                       CH.sub.3                                                                              g                                           h                                                                                        ##STR7##       CH.sub.3                                                                              h                                           ______________________________________                                    

According to the article "Anodic Methoxylation of Alkylbenzenes" by K.Sasaki, H. Urata, K. Uneyama and S. Nagaura in Electrochimica Acta,1967, Vol. 12, pp. 137-146, toluene (Ia) is converted on platinumelectrodes in methanol to benzaldehyde-dimethylacetal (IIa) (or, afterhydrolysis, to benzaldehyde itself) and to methyl benzyl ether.According to the data in F. Beck, Elektroorganische Chemie, VerlagChemie-Weinheim, 1974, page 248, the yield of (IIa) is said to be about10% of the theoretical.

Published French patent application No. 2,351,932 describes theelectrochemical oxidation of methylbenzenes, such as toluene (Ia) andxylene (Ib). The following products are obtained in the oxidation oftoluene (Ia) in an electrolyte system composed of methanol and aco-solvent, such as methylene chloride, with the use of acidicsupporting electrolytes and after subsequent hydrolysis: methyl benzylether, benzaldehyde and p-methoxybenzaldehyde, and in addition also,inter alia, o-methoxy-benzaldehyde or p-methoxytoluene. The yields ofbenzaldehyde are 3.6 to 13.2% of the theoretical. In the correspondinganodic oxidation of p-xylene (Ib), the reaction products (no yield dataare given) are methyl-p-xylyl ether, 4-methyl-benzaldehyde,methyl-4-methyl-benzoate and 2-methoxy-4-methyl-benzaldehyde.

According to the article "Anodic Substitution and Addition Reactions" byS. Tsutsumi and K. Koyama in Discussions of the Faraday Society, 1968,No. 45, pp. 247-253, ring-substituted methoxy derivatives of toluene orof tolunitrile are also obtained in the anodic cyanation of toluene onplatinum electrodes in a methanol/NaCN system.

More recently, remarkable increases in the selectivity of theelectrochemical alkoxylation of toluenes substituted in the 4-positionhave also been disclosed.

In the article "Nuclear Cyanation of Methylanisoles" by K. Yoshida, M.Shigi and T. Fueno in J. Org. Chem., 1975, Vol. 40, No. 1, pp. 63-66,the reaction of 4-methoxytoluene (Ic) in a methanolic solution of NaCNor Na acetate is described. In this reaction, 4-(methoxymethyl)-anisoleand, in a material (current) yield of 15% (24%) of the theoretical,anisaldehyde-dimethylacetal (IIc) are also formed, in addition to therespective ring-substitution products.

A. Nilsson, U. Palmquist, T. Pettersson and A. Ronlan, "Methoxylation ofMethyl-substituted Benzene and Anisole Derivatives, and the Synthesis ofAromatic Aldehydes by Anodic Oxidation" in J. Chem. Soc., PerkinTransactions I, 1978, pp. 708-715, have succeeded in methoxylatingp-xylene (Ib) and 4-methoxytoluene (Ic) in methanol with the use ofNaOCH₃ /LiBF₄ or NaOCH₃ supporting electrolytes on a carbon anode atabout 10° C. to give compound (IIb) in a material (current) yield of 57%of the theoretical and compound (IIc) in a material (current) yield of66% of the theoretical.

In Published European application No. 0,011,712 (see also GermanOffenlegungsschrift No. 2,848,397), benzaldehyde-dialkylacetals,substituted in the 4-position, of the general formula ##STR8## aredescribed, wherein R¹, inter alia, can also represent a phenyl radicalor benzyl radical and R² can represent an alkyl radical having 1 to 4 Catoms. These compounds are prepared by electrochemically oxidizingmethylbenzenes (toluenes), substituted in the 4-position, of the generalformula ##STR9## in the presence of an alcohol R² --O--H (R¹ and R²having the meaning indicated above) and of a supporting salt. Examplesof suitable supporting salts are: fluorides such as KF, tetrafluoboratessuch as Et₄ NBF₄ (where Et=ethyl), perchlorates such as Et₄ NClO₄,sulfates such as Et₄ NSO₄ Et, alcoholates such as NaOCH₃ and hydroxidessuch as KOH. The quantitative proportions of the components should bebetween about 5 and 50% by weight of the substituted methylbenzenes,between about 50 and 95% by weight of the alcohol and between about 0.5and 15% by weight of the supporting salt. Graphite, graphite-filledplastics and noble metals are mentioned as the anode materials, andgraphite, iron, steel, lead and noble metals are mentioned as thecathode materials. The current densities are from 1 to 20 A/dm², and theelectrolysis temperature is between about 0° and 60° C. In detail, theanodic methoxylation or ethoxylation, respectively, of p-xylene (Ib)gives a material yield (current yield) of 32% (18%) of the theoreticalof 4-methyl-benzaldehyde-dimethylacetal (IIb), that of 4-methoxytoluene(Ic) gives 42.4% (22%) to 73.1% (56.5%) of anisaldehyde-dimethylacetal(IIc) or 53.4% (-) of anisaldehyde-diethylacetal (IIc'), that of4-benzyloxytoluene (Id) gives 62.1% (47.9%) of4-benzyloxy-benzaldehyde-dimethylacetal (IId), that of 4-allyloxytoluene(Ie) gives 36.3% (10.8%) of 4-allyloxy-benzaldehyde-dimethylacetal(IIe), that of 4-phenoxy toluene (If) gives 39.2% (14.3%) of4-phenoxy-benzaldehyde-dimethylacetal (IIf), that of 4-t-butoxytoluene(Ig) gives 52.5% (19.2%) of 4-t-butoxy-benzaldehyde-dimethylacetal (IIg)and that of 4-N,N-dimethylamino-carboxyl-toluene gives 40.4% (-) of4-(N,N-dimethylamino-carboxyl)-benzaldehyde-dimethylacetal.

In the process for the preparation of substitutedbenzaldehyde-dialkylacetals according to Published European applicationNo. 0,012,240, inter alia, methylbenzenes (toluenes), substituted in the4-position, of the general formula ##STR10## in which R¹ can, interalia, represent an aryloxy or aralkoxy radical, are electrochemicallyoxidized in solution in an alcohol of the formula R² --O--H (R² =alkyl)and in the presence of a supporting salt at a current density of 0.1 to50 A/dm². The aryloxy and aralkoxy radicals mentioned are, inter alia,phenoxy, naphthyloxy, anthryloxy, benzyloxy and 2-phenyl-ethoxy,according to which p-phenoxytoluene (=1-methyl-4-phenoxy-benzene) shouldthen, inter alia, also be included with the methylbenzenes substitutedin the 4-position. Suitable supporting salts includetetraethylammonium-p-toluene-sulfonate, tetraethylammonium-ethyl-sulfateor tetramethylammonium dimethyl phosphate. Graphite, lead dioxide andnoble metals are mentioned as anode materials, and copper, nickel,steel, platinum and graphite are mentioned as cathode materials. Indetail, the anodic methoxylation of p-xylene (Ib) gives a material yield(current yield) of 64% (-) of 4-methylbenzaldehyde-dimethylacetal (IIb),that of 4-methoxytoluene (Ic) gives 67% (71%) to 95% (-) of4-methoxybenzaldehyde-dimethylacetal and that of 4-t-butoxytoluene (Ig)gives 55% (-) to 92% (-) of 4-t-butoxybenzaldehyde-dimethylacetal (IIg).

None of these numerous publications, however, contains even theslightest hint to the effect that it would be possible to alkoxylatearomatic compounds which have 2 methyl groups located on an aromaticsystem in the molecule, to simultaneously alkoxylate both methyl groupsto give the corresponding aromatic dialdehyde-bis-dialkylacetals. Thus,for example, as described by A. Nilsson et al. (cited above), the anodicoxidation of p-xylene (Ib), depending on the experimental conditions(page 709, bottom of right-hand column) either leads via a nuclearmethoxylation to1-methyl-1-methoxy-4-methyl-4-methoxy-cyclohexa-2,5-diene (III) or leadsvia a side-chain methoxylation to (4-methyl-benzyl)-methyl-ether (IV),to 4-methylbenzaldehyde-dimethylacetal (IIb) or, in the last oxidationstage, to the ortho-ester of p-toluic acid (V): ##STR11##

The even more recent publications (German Offenlegungsschrift No.2,848,397, European application No. 0,011,712 or European applicationNo. 0,012,240) likewise do not contain any hint to the effect that, inthe anodic methoxylation of, e.g., p-xylene (Ib), even traces ofterephthaldialdehyde-bis-dimethylacetal (VI) ##STR12## could be formed.Even with the most sensitive analytical methods, such as a combinationof gas chromatography and mass spectroscopy according to PublishedFrench application No. 2,351,932, it was not possible to detect eitherthe dialdehyde or the corresponding bis-dimethylacetal in this reaction.

These findings coincide with the results in an article "Anodic Oxidationof Arylcyclopropanes" by T. Shono and Y. Matsumura in J. Org. Chem.,1970, Vol. 35, No. 12, pp. 4157-4160, according to which either anα-methoxylation to give 2-(p-tolyl)-2-methoxypropane, or the secondaryproduct p-isopropenyl-toluene obtained therefrom, or an α'-methoxylationto give (p-i-propyl-benzyl)-methyl-ether takes place in the anodicmethoxylation of 4-i-propyltoluene. Accordingly, once a methoxy grouphas entered into the α-position, a possible subsequent α'-substitutionin the same molecule is completely inhibited.

SUMMARY OF THE INVENTION

It is the therefore an object of the present invention to provide aprocess for the preparation of bis-dimethylacetals.

It is also an object of the invention to produce a symmetricalbis-dimethylacetal, namely, 4,4'-diphenylether-dialdehyde-bis-dimethylacetal, by an anodic methoxylationreaction.

In accomplishing the foregoing objects, there has been provided inaccordance with the present invention a new compound, 4,4'-diphenylether-dialdehyde-bis-dimethylacetal, having the formula ##STR13##

In accordance with another aspect of the invention, there has beenprovided a process for the preparation of the foregoing compound whichcomprises the step of anodically oxidizing di-p-tolyl ether of theformula ##STR14## in the presence of methanol and a supportingelectrolyte. Preferably, the supporting electrolyte comprises NaOCH₃,KOH, KPF₆, CsF, NaBF₄, LiBF₄, tetraethylammonium p-toluenesulfonate, H₂SO₄, CH₃ OSO₃ H, or a mixture thereof. Most preferably, the supportingelectrolyte is a type such that, after the electrolysis has ended, itcan be converted into a compound which is insoluble or does notdissociate in methanol.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of preferred embodimentswhich follows.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention relates to the new compound 4,4'-diphenylether-dialdehyde-bis-dimethylacetal (VII) ##STR15##

This compound is prepared by anodically oxidizing di-p-tolyl ether(VIII) in the presence of methanol and of a conducting electrolyte. Thereaction can be represented as follows: ##STR16##

For the first time in a surprising manner, contrary to the previousfindings and expectations, this enables an aromatic compound having 2methyl groups located on an aromatic system to be anodicallymethoxylated on both sides, that is partially and at the same timesymmetrically, in such a way that the compound (VII) is obtained as thereaction product. The process for the preparation of this compound (VII)represents a modification of the process described by A. Nilsson et al.(cited above).

The process according to the invention can be carried out in the knownpressure cells or flow cells of undivided construction, optionally withstirring or with pumped circulation of the electrolyte. The flowvelocity of the electrolyte is of rather subordinate importance and canbe varied between about 5 cm/second and 10 m/second withoutsignificantly impairing the reaction. The anode materials used are thematerials customary in organic electrochemistry, such as platinum, leaddioxide, graphite or vitreous carbon. Metal-free anodes are preferred.The cathode materials used are the customary solids, such as steel,nickel or graphite, and materials having a low cathodic hydrogenovervoltage are preferred. The form of the anodes and cathodes is ofminor importance and, for example, plates, rods, spheres or evenstructured shapes can be used.

The anode current densities can be varied within the range from about 5mA/cm² (0.5 A/dm²) up to about 500 mA/cm² (50 A/dm²), and the rangebetween about 50 and 200 mA/cm², in particular, a range between about 50and 100 mA/cm², is preferred.

The supporting electrolytes which can be used for the process accordingto the invention are those customary in organic electrochemistry. Theseinclude, in particular, NaOCH₃, KOH, KPF₆, CsF, NaBF₄, LiBF₄,tetraethylammonium p-toluenesulfonate, H₂ SO₄ or CH₃ OSO₃ H.Perchlorates, such as, for example, LiClO₄, are unsuitable in practice.The supporting electrolytes can be used either individually or as amixture. Those supporting electrolytes are preferred in the processaccording to the invention which, after the electrolysis has ended, canbe converted into an insoluble or non-dissociating compound by theaddition of a suitable auxiliary substance which is as anhydrous aspossible. For example, supporting electrolytes, such as H₂ SO₄ orNaOCH₃, can be converted into such an "inactive" compound withstoichiometric quantities of NaOCH₃ or H₂ SO₄, respectively. Thispossibility of "inactivating" the supporting electrolyte is especiallyimportant whenever, after the electrolysis has ended, the reactionmixture containing the bis-dimethylacetal as an intermediate is to bedirectly processed further to give end products which are to be modifiedchemically. In this case, additional working-up stages can then beomitted. This process variant is particularly advantageous in thehomogeneous hydrogenation, described below, as a possibility for directfurther processing of the bis-dimethylacetal. The concentration of thesupporting electrolyte or electrolytes is advantageously within therange of from about 0.01 to 20% by weight, preferably from about 0.05 to5% by weight, relative to the total electrolyte.

The reactant in the process according to the invention, namelydi-p-tolyl ether, can be present in the electrolyte in a concentrationof from about 1 to 40% by weight, relative to the total electrolyte. Thesolvent used for the process according to the invention is methanol inthe anhydrous form, or alternatively, a technical grade having a smallwater content of, for example, about 0.2 to 0.3% by weight. Theelectrolysis temperature can vary within wide limits, and in general itshould be between about 10° C. and 65° C., that is to say approximatelybetween room temperature and the boiling point of the electrolytemixture.

The reaction batch can be worked up by one of the conventionalprocedures, for example, by distillation or filtration. Preferably,however, it should be carried out with exclusion of moisture, so thatthe bis-dimethylacetal formed according to the invention is nothydrolyzed to the bis-aldehyde. The supporting electrolytes andsolvents, which are obtained by such working-up of the reaction batchand are thus recoverable, can be re-used in subsequent batches not onlywithout problems, but with particular advantage. In one variant of theprocess according to the invention, as already described above, thesupporting electrolytes are converted into "inactive compounds" so thatthe reaction mixture can be directly processed further to give furtherproducts.

In the process according to the invention, in particular at a currentconversion of about 8 to 12 Faradays/mole, almost exclusively4,4'-diphenyl etherdialdehyde-bis-dimethylacetal is obtained from thedi-p-tolyl ether. By contrast, at a current conversion of about 4Faradays/mole, only a very inhomogeneous product mixture of 6 compounds,namely, unconverted di-p-tolyl ether and the side-chain substitutionproducts monomethoxy-, symmetrical dimethoxy-, asymmetrical dimethoxy-,trimethoxy- and tetramethoxy-di-p-tolyl ether, is obtained.

The process according to the invention, by producing a high degree ofupgrading, opens up a new route to an industrially important compound,namely 4,4'-bis-methoxy-methyl-diphenyl ether (IX). ##STR17## Thiscompound is of importance, in particular, in the preparation of aromaticpolyethers from hydroxymethyl-diaryl ethers or alkoxymethyl-diarylethers. (See, for example, German Pat. No. 1,252,903 or U.S. Pat. No.3,316,186.) These aromatic polyethers are suitable as a binder in thepreparation of casting compositions or as an adhesive in the productionof laminates. Another known use of the compound (IX) is its reaction,described in German Offenlegungsschrift No. 2,065,732 (=U.S. Pat. No.3,867,147) with aromatic diazonium compounds to give light-sensitivecondensation products from these two components. The preparation of (IX)by homogeneous catalytic hydrogenolysis of 4,4'-diphenylether-dialdehyde-dimethylacetal, the compound according to theinvention, in methanol in the presence of a soluble cobalt carbonylcatalyst modified with organic nitrogen bases is described in detail inGerman patent application No. P30 48 993.1, corresponding to U.S.application Ser. No. 333,012, filed concurrently herewith, with thetitle "Process for the Catalytic Hydrogenolysis of p-SubstitutedBenzaldehyde-dimethylacetals to Produce the Corresponding Benzyl MethylEther Derivatives.

In the examples which follow, unless stated otherwise, % data are byweight. Parts by weight have the same relationship to parts by volume asthe kg to the dm³.

EXAMPLE 1

The electrolytic cell used is an undivided flow cell of polyethylenewith minimized electrode spacing, wherein the geometrical cathode andanode areas are 0.1 m², the electrode separation is 1 mm and the flowvelocity of the electrolyte is 0.8 m/second. Devices for keeping thetemperature constant, for taking samples, or the like, are located inthe electrolyte circulation outside the cell. Commercially availableapparatus grade graphite impregnated with synthetic resin (Diabon-N fromSigri Elektrographit GmbH, Meitingen) is used as the anode, and acommercially available electrolysis graphite (EH type from the samemanufacturer) is used as the cathode. The electrolyte is composed of4,040 parts by weight (5,100 parts by volume) of technical grademethanol, 29.2 parts by weight of incompletely dissolved NaBF₄ and 595parts by weight of di-p-tolyl ether. The molar ratios are 0.266 mole ofconducting salt per 3 moles of reactant. The other reaction conditionsare: a temperature of 45° C., a current density of 50 mA/cm², a cellcurrent of 50 A, a cell voltage of 7.3 to 7.7 V and a current conversionof 9.2 Faradays/mole corresponding to 740 ampere-hours (=115%).

After the reaction has ended, the methanol is distilled off, withexclusion of moisture, over a packed column (main fraction under normalpressure, and the remainder in a vacuum of about 20 to 40 mm Hg), thedistillation residue is stirred with 2,000 parts by volume of drydiethyl ether, and the precipitated conducting salt is filtered off withsuction. After the residue is washed with dry diethyl ether and isdried, 27.6 g (=94.5% of the theoretical) of the conducting salt arerecovered. The diethyl ether is removed from the reaction product bydistillation, and 829.3 g of distillable constituents are separated offfrom 65.2 g of non-volatile, resinous constituents in a thin-layerevaporator (oil circulation temperature 250° C., oil pump vacuum 0.01 to0.2 mm Hg).

The gas-chromatographic analysis (GC) of the distillate [10% strength inmethanol, 1.5 m of Silikon OV 225: 25% of phenyl, 25% of cyanopropyl,methyl on 80 to 100 mesh Chromosorb G AW DMS (manufacturer: JohnManville Products), 60 ml of He/minute, 80° C. start, 8°/minute] shows90.4 area-% of compound (VIII), namely 4,4'-diphenylether-dialdehyde-bis-dimethylacetal. In addition to the mass spectrum(molecular mass 318), the elementary analysis

found: C, 67.4%; H, 6.8%; O, 24.4%.

calculated: C, 68.1%; H, 6.7%; O, 25.2%

and the highly symmetrical ¹ H-NMR spectrum (60 Mhz in CDCl₃) with

C--H (acetal) 12 H, S, 3.3 ppm,

C--H (aldehyde) 2 H, S, 5.32 ppm and

C--H (aromatic, 1,4-disubstituted) 8 H, AA'BB' system, 7.2 ppm

prove the presence of the bis-dimethylacetal according to the invention.

EXAMPLE 2

The electrolytic cell used is an undivided flow cell of polyethylenewith a minimized electrode separation. The cell contains 1 VA steel endcathode, 1 end anode of vitreous carbon (Sigradur K) and 4 bipolarcenter electrodes of vitreous carbon/VA steel (manufacturer of thecommercially available electrodes: Sigri Elektrodengraphit GmbH,Meitingen). The electrode separation is defined by 1 mm thickpolyethylene grids, the anode and cathode areas are each 5×255 cm²,i.e., a total of 1,275 cm² each, and the flow velocity is set to 0.8m/second. Devices for keeping the temperature constant, for takingsamples, and the like, are located in the electrolyte circulationoutside the cell. The electrolyte is composed of 12,750 parts by volumeof technical grade methanol, 30 parts by volume of concentrated H₂ SO₄and 595 parts by weight of di-p-tolyl ether. The other reactionconditions are: a temperature of 18° C., a current density of 100mA/cm², a cell current of 25.5 A, a cell voltage of 34 to 38 V and acurrent conversion of 10.6 Faradays/mole corresponding to 852ampere-hours (=132%).

After the reaction has ended, the conducting electrolyte is neutralizedwith 61 parts by weight of NaOCH₃ (in methanol solution) and theelectrolyte is worked up in accordance with the instructions of Example1 and analyzed by gas chromatography, according to which thehigh-boiling compounds, which can be separated off, contain 89.7% ofcompound (VII).

After clarification by filtration, the neutralized electrolyte, withoutthe working-up indicated in Example 1, can be successfully processedfurther directly, for example, in the homogeneous catalytichydrogenolysis reaction described in the above-mentioned U.S. patentapplication Ser. No. 333,012, filed concurrently herewith.

What is claimed is:
 1. 4,4'-Diphenyl ether-dialdehyde-bis-dimethylacetal, having the formula ##STR18## 